[M_a_t_t]

You could just wait til you have the camper (didn't see it in this thread so I don't know the details), then put a thin board loosely attached to the bottom (using something like an F-clamp) that would represent the lowered body. Then pull the camper around hitting some bumps. If you "bottom out" the axle then the board will move. This way you can test how using a similar spring rate while only lowering it would affect it.

I've got my van lowered 2" and I tow a car trailer every once in awhile (up to 4200 lbs). The van is only rated for 5500 lbs so I'm close to the limit. Unless you are maxing your tongue weight capacity out then I think you will be fine.